Certain heat treat processes and other related industrial heating applications such as brazing and sintering have, at least for certain applications, been traditionally conducted in industrial vacuum furnaces. Standard vacuum furnaces are constructed with a double wall configured in a cylindrical or spherical shape and employ a water jacket between the walls for cooling. This type of furnace is considerably more expensive than the conventional, box type standard atmosphere furnace which operates at atmospheric pressure and which is constructed by fibrous insulation attached to a furnace casing of sheet steel. Because of the water jacket construction in vacuum furnaces, heating is conducted in a vacuum furnace by means of graphite bars or electrodes surrounding the work and connected to a source of electrical power by electrical feedthroughs extending through the casing. In contrast, standard atmosphere furnaces typically use gas fired burners for heating which is a more cost efficient form of energy. Because the atmosphere within a standard atmosphere furnace must be precisely controlled, high temperature, standard atmosphere furnaces indirectly heat the work (i.e. heat by radiation for temperatures in excess of about 1500.degree. F.) by means of burners which fire their products of combustion into radiant tubes which extend into the furnace. The radiant tubes may be either of the single-pass or the single-ended, double-pass type and the prior art is replete with numerous arrangements and configurations of such radiant tubes.
Until recently, components within standard atmosphere furnaces constructed of high alloyed steel limited the temperature at which such furnaces operated to a maximum of about 1750.degree.-1850.degree. F. Standard atmosphere furnaces which operate at such temperatures are referred to today as "high temperature" furnaces. Several years ago, Surface Combustion, Inc., under a contract funded by GRI developed an ultra-high temperature, standard atmosphere furnace now marketed by Surface under the brand name or trademark "ULTRACASE". Reference should be had to GRI U.S. Pat. No. 4,802,844 for a discussion of the deleterious effects temperature has on the life of steel alloys when the temperature begins to exceed 1850.degree. F. In the '844 patent, a retractable hearth lift mechanism is employed to permit the furnace to operate at temperatures of about 2000.degree. F. The limiting factor preventing furnace temperature in excess of about 2050.degree. F., except for short durations, is the life of the high alloy steel radiant tube, i.e. thermal fatigue. That is, standard atmosphere furnace construction techniques using various ceramic types of insulation applied to a standard furnace casing sufficiently insulates the furnace to permit it be operated at temperatures in excess of 2000.degree. F., i.e. at temperatures in the ranges approaching or equal to that utilized in vacuum furnace treatments. The limiting factor preventing higher furnace temperatures in gas fired, standard atmosphere furnaces is the radiant tube.
GRI Report 88/0159 discusses in depth the feasibility of using a "soft" vacuum defined as 10-250 torr coupled with furnace purging in a conventional atmosphere type furnace to perform those types of heat treat and heat treat type processes heretofore accomplished in vacuum furnaces where the work is heated in a "hard" vacuum below 10.sup.-1 torr. The report concludes that at "soft" vacuum levels of about 100 torr and at elevated temperatures of between about 1950.degree.-2350.degree. F. it is possible to metallurgically perform a number of such processes, which are detailed in the report. Thus, the report maintains that it is feasible to use a cost effective, modified atmosphere type furnace construction at high temperature under soft vacuum levels to perform certain types of industrial heat processes heretofore not thought possible in standard atmosphere furnaces. Standard atmosphere furnaces can be heated electrically. However, because of exposure to various furnace atmospheres (not present in "hard vacuum" furnaces), electrical heating elements have to be shielded, i.e. placed within radiant heat tubes. More importantly, operating cost efficiencies dictate that gas burners be used. Again, this means, because of furnace atmosphere composition requirements, radiant tubes.
Recently, ceramic radiant tubes constructed of silicon carbide have been introduced into the furnace art as replacements for steel alloy radiant tubes. While their commercial acceptance is not widespread, ceramic radiant tubes have much higher tensile strength at elevated temperatures (i.e. the temperature ranges under consideration) when compared to steel alloy radiant tubes. While investigation of the suitability of ceramic radiant tubes to the "soft" vacuum furnace application under discussion is still continuing, it is known that ceramic radiant tubes are extremely brittle. Special arrangements have to be undertaken when ceramic tubes are used in a horizontal placement position to minimize stress placed on the tubes. Furthermore, because of the elevated temperatures at which the furnace is operated, special consideration has to be given to the heat flux imparted by the tubes to the work so as to uniformly heat the work. In this regard, it is known by Surface Combustion, Inc. to position radiant tubes uniformly about work centered on the centerline of a cylindrical furnace similar to that used herein and operated under vacuum conditions. With respect to conventional vacuum furnaces, electric heating elements have been positioned to circumscribe the work and temperature uniformity is not as critical a problem as it is when point or line sources of radiant heat are used to heat the work by radiation. Finally, the ceramic radiant tube must be secured to the steel furnace casing in such a way which permits the tube to expand without incurring undue stress and at the same time, seal the tube so that leakage of deleterious oxygen into the furnace chamber which is under a vacuum does not occur.
With respect to vacuum sealing, it is well known in the vacuum furnace art to seal the furnace door by means of elastomer seals which are kept cool by a water jacket. It is also known, for example, by Surface Combustion's internal heat exchanger tubes marketed under the brand name or trademark INTRA-KOOL, to seal the tube at the casing by means of an elastomer seal adjacent a water jacket.